Abstract
The thesis is concerned with evaluating the effectiveness of cut-off walls beneath water retaining structures, primarily earth-embankment dams. An introductory chapter considers underseepage control measures in general and the types of cut-off wall used in Civil Engineering practice are described in detail. Cut-off wall effectiveness is defined in terms of flow and head efficiency. A comprehensive review of previous work is undertaken, which includes analytical solutions and performance data obtained by analytical and experimental means. Additional analytical methods are developed and a solution for the important case of anisotropy within a foundation is presented. As a first stage in obtaining performance data for cut-off walls, various hydraulic models were built and tested, the most effective of these being the single tank horizontal flow model. The scope of the work was extended through the use of electrical analogue models; the continuous conductor model proved suitable only for the testing of perfect cut-offs while the resistor network model had wider application. Supplementary work was undertaken by numerical analysis using the finite difference method of approach and a digital computer to achieve a solution. Data from these experimental and theoretical investigations are presented in the form of relationships correlating head efficiency with cut-off wall penetration ratio and depending on the type of wall simulated, either the degree of imperfection or the permeability ratio. Of the various methods used, the electrical resistor network analogue model is considered to be the most useful. Case histories providing performance data on installed cut-off walls are presented and analysed. The main points of the work are summarised and on the basis of data resulting from the investigation some broad conclusions are drawn. Design recommendations are made and an area of future study is identified.